Electrical oscillation generator



Aug. 13, 1935.

N. M. RUST ELECTRICAL OSCILLATION GENERATOR Filed April 26, 1933 2 Sheets-Sheet l FIIIIII:

- wil INVENTOR ATTORNEY Aug. 13, 1935. N. M. RUST 2,011,291

ELECTRICAL OSCILLATION GENERATOR I Filed April 26, 1935 2 Sheets-Sheet INVENTOR A/OEL Y eusr ATTORNEY Patented Aug. 13, 1935 UNITED STATES ELECTRICAL OSGILLATION GENERATOR Noel Meyer Rust, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application April 26,1933, Serial No. 668,136 In Great Britain May 6, 1932 6 Claims.

This invention relates to electrical oscillation generators, and more particularly, to valve generators of the kind wherein oscillation generation 1 any external feed-back circuit, are well known and will be referred to in the present specification as negative resistance valves. The so-called screened grid valve is an example of a negative resistance-valve and, as is well known, for certain conditions of applied potential the characteristic connecting anode current (ordinates) with anode volts (abscissa) shows a downward slope over a portion of its length, i. e. within certain limits of applied potentials the screened grid valve exhibits negative resistance effects inherently. The negative resistance properties of negative resistance valves have previously been employed in oscillation generators and the principal object of the present invention-is to provide an oscillation generator of the kind wherein generation is obtained by virtue of such negative resistance effects and which shall be adapted to gen erate oscillations of great constancy of-amplitude and frequency. The present invention maythus be utilized to provide an oscillation generator adapted to generate any desired frequency within l a predetermined range of frequencies and wherein great stability as regards frequency and amplitude is obtained. 7 i

According to this invention, an oscillation generator of the kind referred to comprises a negative resistance valve, resonant means associated with said valve for determining the frequency of oscillations generatedthereby and means actuated in dependence upon the oscillatory output from said valve for applying-controlling bias potentials to said valve whereby the amount of negative resistance exhibited by said valve is automatically controlled in a direction to produce substantial constancy of output. i

The nature of the present invention may be explained by reference to and comparison with a known dynatron or negative resistance valve oscillator comprising a screened grid valve having a tuned circuit connected between its anode 5 and the positive terminal of the anode battery,

suitable fixed bias potentials being applied to the two grids so as to cause the valve to exhibit negative resistance effects. It will be apparent that oscillations will begenerated in all conditions in 5 which the negative conductance ofthe anode circuit of the valve is greater in magntiude than the dynamic conductance of the external circuit including the tuned circuit, and oscillations will be built up until the average value of the negative conductance taken over the range of working is equal to the said dynamic conductance. An examination of the anode current-anode voltage and anode conductance-anode voltage characteristic curves of a screened grid valve will show that with the known circuit described wherein the bias potentials applied to the two grids are constant, the valve will operate well beyond the relatively short linear portion of the downwardly sloping portion of the anode current-anode voltage characteristic curve and in consequence the oscillations generated will not be of high constancy as regards frequency and amplitude. The present invention meets this difiiculty by utilizing a rectified component obtained from the output circuit of the valve automatically to adjust the bias potential applied to said valve in such manner that in oscillating the valve is maintainedworking' between those limits on the negative resistance portion of the characteristic curve between which the said characteristic curve is substantially straight.

. The invention is illustrated in the accompanying diagrammatic drawings wherein Figures 1 to 5 show five embodiments of the invention.

Referring to Figure 1 which shows one circuit arrangement in accordance with the invention, the above described known form of dynatron oscillator is modified by connecting a diode I or other suitable rectifier in a circuit extending from the anode 2 of the screened grid valve 3 to the cathode 4 thereof, this circuit including a resistance 5 shunted by a condenser E the said combination of resistance and capacitybeing connected between the anode l of the diode and the cathode of the screened grid valve. The control or inner grid 8 of the screened grid valve is connected to an adjustable tapping point 9 upon the resistance 5, and the cathode 4 of the said valve is also connected to this resistance. Fixed bias is applied to the outer or screening grid ID by means of a battery H in series with the anode battery l2, the fixed bias applied to the screen grid being larger than the potential applied to the anode. l3, I4 is a tuned circuit connected between the anode of the valve 3 and the junction point of the batteries ll, l2. a

In a slight modification of the last described circuit and which is illustrated in Figure 2, acircuit consisting of a diode l a preferably adjustable battery [5 and a resistance It shunted by a capa-city i I is included in series between the anode 2 of the screened grid valve and the cathode 4 thereof, the arrangement of the tuned circuit anode battery and battery for providing potential to the screening grid additional to that pro- Vided for the anode, being all as previously described. The lead to the inner grid 8 contains a battery. 58 of appropriate voltage so that the said inner grid is. biased'negatively with respect to the cathode.

The use of a diode for effecting the necessary control has two disadvantages:First, the control pulses applied by the diode to'the screen grid valve produce a slight damping efiect on the tuned circuit and hence gives rise to some slight distortion, and secondly, the amount of bias compensation necessary is dependent upon the difierence be tween the dynamic conductance of the circuit and that of the valve in its uncompensated state. As the former factor changes with the setting of the tuned circuit it will be apparent that a difierent compensation will be required for each setting. As the bias voltage is obtained from the rectifier, larger amplitude swings will be required to produce the necessary compensating bias as the dif ference between the conductances increases, or, toput the matter in another way',the difference between the peak voltage and that voltage at which the rectifier just starts to conduct (this may be termed the excess voltage) will alter, and, therefore, if the excess voltage is relatively large as compared to the total swing, the constancy of amplitude may be more or less seriously vitiated as the setting of the tuned circuit is altered. These disadvantages can be avoided by employing a triode or other amplifier valve in place of the diode. One arrangement of this kind is illustrated in Figure 3 and may be regarded as a modification of the embodiment shown in Figure 2, the modification consisting mainly in replacing the diode I by a triode Hi the grid 20 of said triode being connected to the anode of the screened grid valve and the cathode 2i of said triode being connected to a preferably adjustable tapping point 22. upon a battery 2;: whose negative terminal is connected to the positive terminal of the source l2 of anode potential, 1. e. to the battery end of the tuned circuit l3, M. The anode of the triode is connected through a battery 24 to the capacity shunted resistance l6 whose other end is connected to the cathode of the screened grid valve. If a triode of the correct constants be employed and the bias potential applied thereto backs it oh as required, a relatively small excess voltage on the grid of'said triode will produce the necessary control bias on the screened grid valve.

In a; modification of the last described embodiment which is illustrated in Figure 4, two triodes 2'5,v 26, in pushpull are substituted for the single triode [9 of Figure 3. The grids 2?, 28 of the two triodes are connected to opposite ends of the tuned circuit 13, M and the cathode point 29 is connected to a preferably adjustable tapping point 30' upon a battery 3! whose negative terminal is connected to the mid-point 32 of the inductance of said tuned circuit, i. e. to the electrical center of said tuned circuit. The two anodes 33, 3d, of the triodes are connected together and through a common anode battery E i whose negative termina-l-is connected to one end of the resistance It as before.

In a further modification illustrated in Figure 5' what is herein termed a double-diode-amplifier valve is employed as control valve. The

double-diode amplifier valve is shown at 35 and is a valve having a heater 36, a cathode 31 heated thereby, a pair of diode anodes 38, 39 which cooperate with said cathode to constitute a full wave rectifier, a control grid 40 and what is herein termed a valve anode All, the two latter electrodes cooperating with the cathode after the manner of an ordinary thermionic valve. In Figure 5 the valve 3 is shown as of the indirectly heated cathode type and, as will be seen, voltage from the tuned circuit it, M is applied via a coil 4-2 to the diode anodes 38, 39 and the center point of coil 42 is connected through an adjustable bias battery 43 to the grid 40 and also through said battery and a-resistance shunted condenser combination M, to the cathode 31. Anode potential to the anode H is supplied from battery H via a load (represented at LR by an adjustable resistance) and an indicating meter IM. The cathode 3? is connected to the heater 36 through a resistance shunted condenser combination 46, ll, and also through the bias battery E3 to the control grid of the valve 3.

In the arrangements described, as soon as the oscillations of the screened grid valve tend to increase beyond the proper value av rectified pulse is obtained from the control valve (diode, for example) and this rectified pulse charges the resistance shunted condenser. This in turn biases the appropriate grid of the screened grid valve in a. direction to cause said valve to oscillate morefeebly and it will be apparent that if the circuit constants be suitably chosen the bias onthescreened grid valve will be automatically adjusted to such value that the said valve does not swing beyond? the predetermined desired limits on its characteristic. If, for any purpose, it be required to provide for adjustment of the. amplitude of the oscillations generated this may be obtained, for example, by varying the anode voltage applied to the diode (where diode control is employed) or by varying the grid bias on the control triode or triodes where three electrode valves are employed for control,

,The'frequency of the oscillations generated are, of course, maintained most accurately at the natural frequency of the tuned circuit when. the: oscillation amplitude is restricted by making the. negative conductance of the screened grid valve at its static setting as nearly as possible equal to thedynamic conductance of the tuned circuit and it will be seen that the circuit arrangements herein described in accordance with the present invention achieve this condition so that a high degree of constancy of frequency is obtained, so far as the eifect of the negative resistance valve is con;- cerned. Of course where very high constancy is required the usual precautions to prevent tem.-- perature changes and so: forth should be taken: since the present invention does not in itself. provide means for avoiding frequency changes due to; this and similar causes. It will also be apparent that substantially sinusoidal oscillations will be produced since the anode volts excursion of the screened grid valve are automatically restricted to regions of linear operation.

I claim:

1. A system in accordance with claim 6, char-- acterized in this that said means for rectifying. oscillatory current derived from the output circuit of said valve includes a pair of triodes.

2. In. combination, a screened grid valve, a tuned circuit in serieswith a. source of anode potential between the anode and cathode of said.

the anode potential to the screen grid of said valve, a circuit including a diode in series with a capacity shunted resistance between the anode and cathode of said valve and a connection between a tapping point on said resistance and the control grid of said valve the whole arrangement being such that the valve operates as a dynatron oscillator.

3. In combination, a screened grid valve, a tuned circuit in series with a source of anode potential connected between the anode and cathode of said valve, means for applyingto the screen grid of said valve a potential higher than the anode potential, a circuit including an electron discharge device in series with a source of potential and with a resistance shunted condenser between the anode and cathode of said valve, and a connection including a source of bias potential between the control grid of said valve and the end of the resistance shunted condenser remote from the cathode.

4. In combination, a screened grid valve, a tuned circuit in series with a source of anode potential connected between the anode and cathode of said valve, means for applying to the screen grid of said valve a potential higher than the anode potential, a circuit including a triode whose grid anode space is included in series with a source of potential and with a resistance shunted capacity, said series circuit being between the anode and cathode of the screened grid valve, the oathode of said triode being connected to a further source of potential whose negative terminal is connected to the positive terminal of the anode source for the screened grid valve, the control grid of said triode beingconnected to the anode of said screened grid device. 7

5. In combination, a screened grid valve, a tuned circuit in series with a source of anode potential connected between the anode and cathode of said valve, means for applying to the screen grid of said valve a potential higher than the anode potential, a circuit including a pair of triodes in push-pull, the grids of said triodes being connected to oppositeends of the tuned circuit associated with the screened grid valve, the anodes of said pair of triodes being connected together and conductively coupled through a series circuit of a source of potential and a resistance shunted condenser positioned in the order named to the cathode of said screened grid valve, the anodes of said pair of triodes being connected to the positive terminal of said last source, and the grid of said screened grid valve being connected to the juncture of said last mentioned source of potential and the resistance shunted condenser.

6. In combination, a negative resistance device comprising an electron discharge device having a cathode, anode, control grid, and screen grid electrode, resonant means in circuit with the anode circuit of said device for controlling the frequency generated thereby, means for applying to the anode and screen electrodes of said device operating potentials of such value that said valve operates over the downwardly sloping portion of its anode-voltage-electron current to anode characteristic, the arrangement being such that the negative conductance of the anode circuit of said valve is greater in magnitude than the dynamic conductance of the external circuit, whereby said device operates as a dynatron oscillator, means including a triode for rectifying oscillatory current derived from the output circuit of said valve and means for applying as bias potential components to the control grid of said device potentials derived from the rectified oscillatory current.

NOEL MEYER RUST. 

